1. Introduction
This invention relates to monitoring the concentration of a component dissolved in solution as its concentration varies with use of the solution.
2. Description of Prior Art
Many solutions contain dissolved components which vary in concentration with use of the solution. For such solutions, it is frequently desirable or necessary to periodically or continuously determine the concentration of a solution component undergoing concentration change. For example, when the concentration of a component undergoing change reaches a given pre-determined level, there may be a need to replenish or remove the ingredient so that its concentration is returned to its approximate initial level. The need for replenishment is especially true with metal plating solutions, typically electroless metal plating solutions such as those illustrated in U.S. Pat. Nos. 3,728,137 (for electroless copper) and 3,719,508 (for electroless nickel), both incorporated herein by reference. For such solutions, as the plating metal is consumed by deposition over a substrate, the properties of the solution are altered such as by a decrease in the plating rate and a change in deposit properties. To avoid such changes, these solutions are periodically replenished whereby consumable ingredients, such as the plating metal, are returned to their initial concentration levels. For solutions used to etch metal, for example, such as those disclosed in U.S. Pat. No. 3,650,957 (for the etching of copper), incorporated herein by reference, when the concentration of the etched metal builds to an undesirably high level, the etch rate may decrease and undesired deposits may form on the etched surface. To prevent these changes in the properties of the etchants, it is desirable to periodically remove a portion of the etched metal by methods known to the art.
Using metal plating solutions for purposes of illustration, it is known that the concentration of the components of such solutions may be periodically monitored by chemical analysis and, in some cases, by continuous analysis using sophisticated analytical instrumentation as illustrated in U.S. Pat. Nos. 3,532,519; 3,951,602; 4,002,786 and 4,096,301, all incorporated herein by reference. Monitoring the concentration of consumable solution components using sophisticated instrumentation frequently requires trained technicians and costly investment in instrumentation. Both are impractical or prohibitively expensive for small plating shops and are frequently unnecessary for larger plating installations where precise control of solution components for good quality plating is not required. For all such installations, it is desirable to provide a low-cost system for effectively monitoring the concentration of critical components in solution which undergo change in concentration during use of the solution which system can be easily used by non-technically trained operators.
One system known in the art for monitoring the concentration of cooper in a copper plating solution is a copper color indicator kit marketed by Shipley Company Inc. which uses a set of color standards, a solution of dye and a simple device called a "color comparator". Using this system, approximate copper concentration in a copper plating solution is determined by taking a 20 ml sample from the plating bath and adding the same to 60 ml of Cuposit.RTM. 7012 copper color indicator, an aqueous solution of methyl orange. The resulting mixture is visibly compared with a set of colored standards using the color comparator, and the replenishment of the plating bath is made in accordance with a replenishment table for the specific copper solution in use.
Although useful, the color comparator suffers several problems in that it requires the operator to take a sample from the plating solution and perform several steps to determine required replenishment. Errors are easily introduced because of the small size of the sample being used and the relatively large volume differences between the color indicator and the solution sample. Therefore, though low in cost, such a system is easily mis-used, especially in the hands of a careless or untrained operator.
Ideally, it would be desirable to provide a plating solution where the concentration of the plating metal may be monitored by observing the solution or by taking a sample of the solution and comparing it to a permanent set of standards without any additional manipulative steps. Such a system would permit a relatively untrained operator to determine concentration of a component undergoing concentration change at any time during use of the solution by looking at the bath or by sampling the bath without the necessity for accurate measurements or other manipulative steps except, perhaps, placing a sample in a simple instrument, without mixing, for better accuracy, if desired.